Indirectly heated cathode (IHC) ion sources operate by supplying a current to a filament disposed behind a cathode. The filament emits thermionic electrons, which are accelerated toward and heat the cathode, in turn causing the cathode to emit electrons into the ion source chamber. The cathode is disposed at one end of the ion source chamber. A repeller is typically disposed on the end of the ion source chamber opposite the cathode. The repeller may be biased so as to repel the electrons, directing them back toward the center of the ion source chamber. In some embodiments, a magnetic field is used to further confine the electrons within the ion source chamber. The electrons cause a plasma to be created. Ions are then extracted from the ion source chamber through an extraction aperture.
One issue associated with IHC ion sources is that the cathode and repeller may have a limited lifetime. The cathode is subjected to bombardment from electrons on its back surface, and by positively charged ions on its front surface. This bombardment results in sputtering, which causes erosion of the cathode.
Further, in some embodiments, tungsten or carbon like material may grow on the surface of the repeller. These deposits may reduce the efficiency of the ion source, or may lead to issues with the plasma, such as, for example, non-uniformity of extracted ribbon ion beams. Further, these deposits may also introduce contaminants into the extracted ion beam and reduce the life of the ion source.
Therefore, an IHC ion source in which material did not build up on the repeller may be beneficial. This IHC ion source may have improved life, performance and beam uniformity.